Lean-burn gasoline engines can be more efficient and thus use less fuel and produce less carbon dioxide than corresponding engines operating under stoichiometric conditions.
To approach to treat engine emissions is to catalytically convert NO to a solid, prototypically barium nitrate, and store it in an emission control device during lean operation. The device is regenerated periodically by briefly shifting engine operation to stoichiometric or rich conditions, under which the barium nitrate becomes released NO that is then reduced. The operating temperature range for the device can be determined by the activity of the catalyst used to form the solid nitrate (defining the lower limit) and the stability of the nitrate under lean conditions (defining the upper limit). A typical range is approximately 200 to 500° C.
Although the device works well initially, its performance typically degrades over time. One reason for this is a slow accumulation of sulfate, derived from the combustion of fuel sulfur, which effectively competes with the nitrate for storage space. The sulfate is more stable than the nitrate, but it can be removed by an occasional exposure to rich conditions at a somewhat higher temperature than that used for normal regeneration of the trap.
The inventors herein, however, have recognized another reason for the degradation in performance of the device. Specifically, there can be a loss in activity of the catalyst used to form the solid nitrate. For example, if the catalyst is platinum supported on a high-surface-area oxide, its loss in activity can result from loss of platinum surface area due to coarsening of the supported particles of platinum. The inventors herein have also recognized that when these different modes of degradation can affect device performance in different ways.
As such, the inventors herein have recognized a disadvantage with prior approaches that monitor degradation. Specifically, the determination of device performance that fails to consider both effects results in inaccurate readings, and thus degraded overall performance. As an example, if device performance is evaluated without considering the interrelationship between degradation due to sulfur, and degradation due to particle coarsening, erroneous determinations of oxygen storage ability, NOx storage ability, and/or conversion efficiency can result.
The above disadvantages with prior approaches are overcome by a method for evaluating performance of an emission control device coupled in an exhaust system of a vehicle driven on the road, the method comprising:
determining a first factor based on a duration during which the emission control device is exposed to a lean air-fuel ratio above a limit value, said limit value determined as a function of temperature in the exhaust system;
determining a second factor based on an amount of sulfur contamination of said emission control device; and
determining a performance value for said emission control device based on said first and second factors.
By determining emission control device performance based both on effects of sulfur contamination, and conditions indicative of platinum particle growth, it is possible to obtain a more accurate estimate that can then be used to improve engine control.
Note that there are various different ways to determine a duration during which the emission control device is exposed to a lean air-fuel ratio above a limit value, with the limit value determined as a function of temperature. For example, the lean air-fuel ratio can be calculated in terms of oxygen partial pressure, and the limit value can also be in terms of oxygen partial pressure.
Note also that the temperature in the exhaust system can be indicative of many different temperature, such as temperature of the emission control device, temperature of exhaust gasses flowing into the emission control device, or a generalized exhaust temperature indicative temperature in the exhaust system.